1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
Typical image forming apparatuses form an image on paper by performing primarily transfer of a toner image to a surface of an endless transfer belt, which is rotatably stretched over a plurality of rollers including a drive roller, a driven roller, and a backup roller, from a photoreceptor drum (an image carrier), and then performing secondary transfer of the toner image to the paper.
Such image forming apparatuses have a secondary transfer roller that presses the transfer belt against the backup roller. When a portion of the transfer belt on which a toner image has been primarily transferred passes through the backup roller, paper is allowed to pass between the transfer belt and the secondary transfer roller, so that the toner image is secondarily transferred on the paper.
In such image forming apparatuses, electrical discharge may occur across the gap between paper and the transfer belt in the vicinity of the entrance of a transfer nip area where the transfer belt is in proximity to the secondary transfer roller, during the secondary transfer of a toner image on the transfer belt to the paper. This causes disorder of the toner image, leading to formation of an abnormal image called toner scattering.
In the case of image formation on heavy and stiff thick paper, the trailing end of the thick paper is bent by being pre-nipped between the transfer belt and the secondary transfer roller, and the trailing end may hit the transfer belt as a reaction of its restoration. The transfer belt inwardly deflects due to the shock of such hitting, resulting in formation of a gap between the thick paper and the transfer belt. Electrical discharge may occur across the gap, leading to formation of the above-described abnormal image.
To reduce the formation of the abnormal image, the transfer belt should be stably driven against an applied load.
For example, in a typical image forming apparatus, a stiff drive belt is provided inside a transfer belt, and the drive belt rotates while supporting the transfer belt to stabilize rotation of the transfer belt (see, for example, Japanese Unexamined Patent Application Publication No. 2010-72311 (JP-A-2010-72311)).
In another image forming apparatus, an endless buffer hoop is disposed between a transfer belt and one of the rollers over which the transfer belt is stretched, so that the transfer belt is less affected by steps of the roller (see, for example, Japanese Unexamined Patent Application Publication No. 2009-63794 (JP-A-2009-63794)).
Unfortunately, in the image forming apparatus disclosed in JP-A-2010-72311, paper which is to be nipped for a transfer is conveyed in a direction substantially perpendicular to a line that connects the axis of the secondary transfer roller with the axis of a counter roller. Accordingly, the paper is not pre-nipped, and thus, the paper is not bent. This means that there is no possibility that the paper hits against the transfer belt with its rebound. In addition, in the apparatus of JP-A-2010-72311, while the drive belt supports the transfer belt in the vicinity of a photoreceptor drum, it is not intended to be a measure against a load on the transfer belt in the vicinity of the secondary transfer roller.
The technology disclosed in JP-A-2009-63794 is to achieve good contact between a cleaning blade for removing foreign substances on a surface of the transfer belt and the transfer belt. Thus, the technology is not intended to be a measure against a load on the transfer belt in the vicinity of the secondary transfer roller.